A two-dimensional h-BN/C2N heterostructure as a promising metal-free photocatalyst for overall water-splitting

The lead-free perovskite-based heterojunction C2N/CsGeI3: an exploration for superior visible-light absorption

Halide perovskites have distinguished themselves among the numerous optoelectronic materials due to their versatile processing technology and exceptional optical response. Unfortunately, their stability and toxicity from heavy metals severely hamper their development, in addition to the challenge of further improving photovoltaic performance. Hence, a lead-free perovskite-based heterojunction, C2N/CsGeI3, is investigated using a hybrid density functional, including electron structures, charge density differences, optical properties and more. The study reveals the presence of a built-in electric field directed from the CsGeI3 to the C2N layer. Moreover, based on the work function, it is confirmed that the electrons are transferred in a Z-scheme mechanism after the CsGeI3 contacts with the C2N layer. Under light irradiation, the construction of the C2N/CsGeI3 heterojunction significantly enhances optical absorption within the range of visible-light wavelengths. Additionally, the impact of interfacial strain on the C2N/CsGeI3 is explored and discussed. These findings not only suggest that the C2N/CsGeI3 heterojunction holds promise for photovoltaic applications but also provide a theoretical insight into lead-free perovskite-based functional materials.

Optoelectronic and photocatalytic behaviour of a type-II GaAlS2/HfS2heterostructure:ab initiostudy

Theoretical examination based on first principle computation has been conducted for van der Waals heterostructure (vdwHS) GaAlS2/HfS2including structural, optoelectronic and photocatalytic characteristics. From the adhesion energy calculation, the AB configuration of GaAlS2/HfS2vdwHS is the most stable. A type-II GaAlS2/HfS2vdwHS is a dynamically and thermally stable structure. The band edge position, projected band, and projected charge densities verify the type-II alignment of GaAlS2/HfS2vdwHS. For GaAlS2/HfS2, GaAlS2is acting as a donor and HfS2is acting as an acceptor ensured by the charge density difference plot. The electron localized function validates the weak van der Waals interaction between GaAlS2and HfS2. The GaAlS2/HfS2vdwHS possess an indirect bandgap of 1.54 eV with notable absorption in the visible range. The findings assure that the GaAlS2/HfS2vdwHS is an efficient photocatalyst for pH 4-8. The band alignment of GaAlS2/HfS2is suitable for Z-scheme charge transfer. The strain influenced band edge suggests that the GaAlS2/HfS2vdwHS remains photocatalytic for strain-4%to+6%in both cases of uniaxial and biaxial strains.

Intriguing type-II g-GeC/AlN bilayer heterostructure for photocatalytic water decomposition and hydrogen production

Adapting two-dimensional (2D) van der Walls bilayer heterostructure is an efficient technique for realizing fascinating properties and playing a key role in solar energy-driven water decomposition schemes. By means of first-principles calculations, this study reveals the intriguing potential of a novel 2D van der Walls hetero-bilayer consisting of GeC and AlN layer in the photocatalytic water splitting method to generate hydrogen. The GeC/AlN heterostructure has an appropriate band gap of 2.05 eV, wherein the band edges are in proper energetic positions to provoke the water redox reaction to generate hydrogen and oxygen. The type-II band alignment of the bilayer facilitates the real-space spontaneous separation of the photogenerated electrons and holes in the different layers, improving the photocatalytic activity significantly. Analysis of the electrostatic potential and the charge density difference unravels the build-up of an inherent electric field at the interface, preventing electron-hole recombination. The ample absorption spectrum of the bilayer from the ultra-violet to the near-infrared region, reaching up to 8.71 × 105/cm, combined with the resiliency to the biaxial strain, points out the excellent photocatalytic performance of the bilayer heterostructure. On top of rendering useful information on the key features of the GeC/AlN hetero-bilayer, the study offers informative details on the experimental design of the van der Walls bilayer heterostructure for solar-to-hydrogen conversion applications.

The Isoelectronic Dopant in the Z-Scheme SnS2/β-As Heterostructure Enhancing Photocatalytic Overall Water Splitting

The catalytic field aims to decrease reaction barriers, accelerate reaction processes, and enhance the selectivity toward a target product. This study uses first-principles calculations to design a modified direct Z-scheme SnS2/β-As heterostructure as a potential photocatalyst for overall water splitting. Our previous investigations have demonstrated that the SnS2/β-As heterostructure can realize a hydrogen evolution reaction (HER) under light, while the oxygen evolution reaction (OER) follows a pathway involving the intermediate HOOH*. Interestingly, by substituting an S atom of SnS2 with a Se or Te atom, the rate-determining step of the OER is significantly reduced from 3.76 eV to 2.56 or 2.22 eV. Moreover, the OER can occur directly without the transition via HOOH*. Isoelectronic doping effectively trades off the adsorption strength of OER intermediates and promotes the OER process. This work highlights the dual benefits of isoelectronic doping, namely lowering the reaction barrier of the rate-determining step and promoting the selectivity of end products. These findings provide insights into the rational design of high-efficiency photocatalysts for water splitting.

Two-Dimensional Ferroelectric C2N/In2Se3 Heterobilayer with Tunable Electronic Property and Photovoltaic Effect

Two-dimensional ferroelectric monolayer materials with reversible spontaneous polarization provide more regulatory dimensions for their relevant van der Waals heterostructures. Using first-principles calculations, we construct the C2N/In2Se3 bilayer heterostructure and study its physical properties as well as the effects of E-field and strain. The results indicate that the intrinsic polarization of the component In2Se3 monoalyer can significantly adjust the electronic properties of the C2N/In2Se3 heterobilayer. When the polarization of the In2Se3 monolayer points to the interface (up-In2Se3), the C2N/In2Se3 bilayer behaves as the type-I indirect band gap heterostructure, while it transforms to the type-II direct band gap heterostructure after reversing the polarization of the In2Se3 monolayer (dp-In2Se3). Furthermore, the two C2N/In2Se3 heterostructures both have enhanced optical absorption in the visible region than the isolated In2Se3 and C2N monolayers. More importantly, the external electric field and strain can easily regulate the electronic properties of the C2N/In2Se3 heterostructures. The power conversion efficiency (PCE) of the type-II C2N/dp-In2Se3 heterostructure is 8.16%, and the electric field of 0.1 V/Å and the strain of -2% can transform the C2N/up-In2Se3 heterostructure into type-II one, conducive to the high PCE up to 24.03 and 24%, respectively. Our proposed C2N/In2Se3 heterostructure is promising in future luminescent and photovoltaic fields, and our findings also provide a strategy for functionalizing 2D monolayer materials by the intrinsic polarization property of ferroelectric materials.

Excellent photocatalytic properties in 2D ZnO/SiC van der Waals hetero-bilayers: water-splitting H2-fuel production

This research unravels the photocatalytic properties of a 2D ZnO/SiC van der Waals hetero-bilayer for potential water-splitting applications by first-principles calculations. Four unique stacking patterns are considered in studying the electronic and optical properties in the presence and absence of biaxial external strain. For pattern-I and II, large negative binding energy and positive phonon frequencies are observed, denoting chemical and mechanical stabilities. Under the HSE-06 pseudo potential, the calculated bandgap value for pattern-I and II reaches 2.86 eV and 2.74 eV, respectively. 2D ZnO/SiC shows a high absorption coefficient (∼10⁵ cm^-1). The absorption peak under biaxial strain could reach ∼3.5 times the peak observed under unstrained conditions. Under strain, a shift from compressive to tensile biaxial strain (-6% to 6%) results in a bandgap decrease from 3.18 eV to 2.52 eV and 3.09 eV to 2.43 eV, for pattern-I and II, respectively. The observed strain-driven kinetic overpotential for 2D ZnO/SiC pattern-I and II easily engenders photocatalytic redox reactions. The excellent mechanical durability and strain-driven large kinetic overpotential suggest 2D ZnO/SiC heterobilayers as a prospective material for water-splitting H₂-fuel production.

Two-dimensional SiC/AlN based type-II van der Waals heterobilayer as a promising photocatalyst for overall water disassociation

Two-dimensional (2D) van der Waals (vdW) heterostructures made by vertical assembling of two different layers have drawn immense attention in the photocatalytic water disassociation process. Herein, we suggest a novel 2D/2D vdW heterobilayer consisting of silicon carbide (SiC) and aluminum nitride (AlN) as an exciting photocatalyst for solar-to-hydrogen conversion reactions using first-principles calculations. Notably, the heterostructure presents an inherent type-II band orientation wherein the photogenic holes and electrons are spatially separated in the SiC layer and the AlN layer, respectively. Our results indicate that the SiC/AlN heterostructure occupies a suitable band-gap of 2.97 eV which straddles the kinetic overpotentials of the hydrogen production reaction and oxygen production reaction. Importantly, the built-in electric field at the interface created by substantial charge transfer prohibits carrier recombination and further improves the photocatalytic performance. The heterostructure has an ample absorption profile ranging from the ultraviolet to the near-infrared regime, while the intensity of the absorption reaches up to 2.16 × 10⁵ cm^-1. In addition, external strain modulates the optical absorption of the heterostructure effectively. This work provides an intriguing insight into the important features of the SiC/AlN heterostructure and renders useful information on the experimental design of a novel vdW heterostructure for solar energy-driven water disassociation with superior efficiency.

Theoretical study on photocatalytic performance of ZnO/C2N heterostructure towards high efficiency water splitting

The construction of van der Waals heterostructures offers effective boosting of the photocatalytic performance of two-dimensional materials. In this study, which uses the first-principles method, the electronic and absorptive properties of an emerging ZnO/C₂N heterostructure are systematically explored to determine the structure’s photocatalytic potential. The results demonstrate that ZnO and C₂N form a type-II band alignment heterostructure with a reduced band gap, and hence superior absorption in the visible region. Furthermore, the band edge positions of a ZnO/C₂N heterostructure meet the requirements for spontaneous water splitting. The ZnO/C₂N heterostructure is known to possess considerably improved carrier mobility, which is advantageous in the separation and migration of carriers. The Gibbs free energy calculation confirms the high catalytic activity of the ZnO/C₂N heterostructure for water-splitting reactions. All the aforementioned properties, including band gap, band edge positions, and optical absorption, can be directly tuned using biaxial lateral strain. A suitable band gap, decent band edge positions, high catalytic activity, and superior carrier mobility thus identify a ZnO/C₂N heterostructure as a prominent potential photocatalyst for water splitting.

Promoted photocarrier separation by dipole engineering in two-dimensional perovskite/C2N van der Waals heterostructures

Due to the aggravation of environmental pollution and the energy crisis, it is urgent to develop and design environment-friendly and efficient photocatalysts for water splitting. van der Waals heterostructures composed of different two-dimensional materials offer an easily accessible way to combine properties of individual materials for applications. Herein, a novel Cs₃Bi₂I₉/C₂N heterostructure is proposed through first-principles calculations. The structural, electronic, and optical properties, as well as the charge transfer mechanism at the interface of Cs₃Bi₂I₉/C₂N are systematically investigated. Due to the difference between the work functions of Cs₃Bi₂I₉ and C₂N monolayers, when they are constructed into heterostructures, redistribution of charge occurs in the whole structure, and some of the charge transfer occurs at the interface due to the formation of an internal electric field. The band structure of Cs₃Bi₂I₉/C₂N has type-II band alignment, and the band edge position as well as the band-gap value of the heterostructure are suitable for visible light water splitting. The in-plane biaxial strain, interfacial spacing, and external electric field can effectively modulate the electronic structure and photocatalytic performance of the heterostructure. Under certain conditions, the heterostructure can be changed from type-II to type-I band alignment, accompanied by the transition from an indirect band-gap semiconductor to a direct band-gap semiconductor. Moreover, the intrinsic anion defect (I vacancy) at different positions, as donor defects, can introduce defect levels near the conduction band edge, which affects the transition of photogenerated carriers in these systems. Our findings provide a theoretical design for strategies to improve the performance of two-dimensional perovskites/C₂N in photocatalytic and optoelectronic applications.

First-Principles Calculations of Two-Dimensional CdO/HfS2 Van der Waals Heterostructure: Direct Z-Scheme Photocatalytic Water Splitting

Using two-dimensional (2D) heterostructure as photocatalyst for water splitting is a popular strategy for the generation of hydrogen. In this investigation, the first-principles calculations are explored to address the electronic performances of the 2D CdO/HfS₂ heterostructure formed by van der Waals (vdW) forces. The CdO/HfS₂ vdW heterostructure has a 1.19 eV indirect bandgap with type-II band alignment. Importantly, the CdO/HfS₂ vdW heterostructure possesses an intrinsic Z-scheme photocatalytic characteristic for water splitting by obtaining decent band edge positions. CdO donates 0.017 electrons to the HfS₂ layer in the heterostructure, inducing a potential drop to further separate the photogenerated electrons and holes across the interface. The CdO/HfS₂ vdW heterostructure also has excellent optical absorption capacity, showing a promising role as a photocatalyst to decompose the water.

Engineering 2D Materials for Photocatalytic Water-Splitting from a Theoretical Perspective

Splitting of water with the help of photocatalysts has gained a strong interest in the scientific community for producing clean energy, thus requiring novel semiconductor materials to achieve high-yield hydrogen production. The emergence of 2D nanoscale materials with remarkable electronic and optical properties has received much attention in this field. Owing to the recent developments in high-end computation and advanced electronic structure theories, first principles studies offer powerful tools to screen photocatalytic systems reliably and efficiently. This review is organized to highlight the essential properties of 2D photocatalysts and the recent advances in the theoretical engineering of 2D materials for the improvement in photocatalytic overall water-splitting. The advancement in the strategies including (i) single-atom catalysts, (ii) defect engineering, (iii) strain engineering, (iv) Janus structures, (v) type-II heterostructures (vi) Z-scheme heterostructures (vii) multilayer configurations (viii) edge-modification in nanoribbons and (ix) the effect of pH in overall water-splitting are summarized to improve the existing problems for a photocatalytic catalytic reaction such as overcoming large overpotential to trigger the water-splitting reactions without using cocatalysts. This review could serve as a bridge between theoretical and experimental research on next-generation 2D photocatalysts.

Two-Dimensional PtS2/MoTe2 van der Waals Heterostructure: An Efficient Potential Photocatalyst for Water Splitting

Recently, the energy shortage has become increasingly prominent, and hydrogen (H₂) energy has attracted extensive attention as a clean resource. Two-dimensional (2D) materials show excellent physical and chemical properties, which demonstrates considerable advantages in the application of photocatalysis compared with traditional materials. In this investigation, based on first-principles methods, 2D PtS₂ and MoTe₂ are selected to combine a heterostructure using van der Waals (vdW) forces, which suggests a type-II band structure to prevent the recombination of the photogenerated charges. Then, the calculated band edge positions reveal the decent ability to develop the redox reaction for water splitting at pH 0. Besides, the potential drop between the PtS₂/MoTe₂ vdW heterostructure interface also can separate the photogenerated electrons and holes induced by the charge density difference of the PtS₂ and MoTe₂ layers. Moreover, the fantastic optical performances of the PtS₂/MoTe₂ vdW heterostructure further explain the promising advanced usage for photocatalytic decomposition of water.

Two-dimensional CdS/SnS2 heterostructure: a highly efficient direct Z-scheme water splitting photocatalyst

A desired water splitting photocatalyst should not only possess a suitable bandgap and band edge position, but also host the spontaneous progress for overall water splitting without the aid of any sacrificial agents. In this work, we propose a two-dimensional CdS/SnS₂ heterostructure (CSHS) as a possible water splitting photocatalyst by first-principles calculations. The CSHS enhances the absorption of visible and infrared light, and the type-II band alignment guarantees the spatial separation of the photoinduced carriers. The induced built-in electric field across the CSHS interface efficiently separates the photoexcited carriers and extends their carrier lifetimes. All these properties make the CSHS a direct Z-scheme system with the hydrogen and oxygen evolution reactions occurring, respectively, at the CdS and SnS₂ layers. More encouragingly, the introduction of a S-vacancy into SnS₂ could effectively lower the overpotential of the oxygen evolution reaction, thus ensuring the overall water redox reaction to be achieved spontaneously under light irradiation. Our findings suggest that the CSHS is a promising water splitting photocatalyst.

MoSSe/Hf(Zr)S2 heterostructures used for efficient Z-scheme photocatalytic water-splitting

HfS2/SMoSe, HfS2/SeMoS, ZrS2/SMoSe, and ZrS2/SeMoS heterostructures are promising overall water-splitting photocatalysts.

Band offset engineering at C2N/MSe2 (M = Mo, W) interfaces

Stacking layered two-dimensional materials in a type-II band alignment block has provided a high-performance method in photocatalytic water-splitting technology. The key parameters in such heterostructure configurations are the valence and conduction band offsets at the interface, which determine the device performance. Here, based on density functional theory calculations, the bandgap and band offsets at C₂N/MSe₂ (M = Mo, W) interfaces have been engineered. The main findings demonstrate that the C₂N monolayer interacts with both MoSe₂ and WSe₂ monolayers through weak van der Waals interactions. These heterostructures possess a narrower indirect bandgap and a typical type-II heterostructure feature, being suitable for promoting the separation of photogenerated electron–hole pairs. The calculated Gibbs free energy of hydrogen adsorption demonstrates a reduction in the overpotential, towards the hydrogen evolution reaction, upon forming heterostructures. To further tune the bandgap values and band offsets of heterostructures, the external perturbations are included through a vertical strain and finite electric field. It is found that both the vertical strain and electric field strongly modulate the bandgap values and the magnitude of the band offsets, while the typical type-II band alignment remains preserved. It is noticeable that the band offset magnitudes of the C₂N/MoSe₂ and C₂N/WSe₂ heterostructures are more sensitive to an external electric field than to a vertical interlayer strain.

Stacking layered two-dimensional materials in a type-II band alignment block has provided a high-performance method in photocatalytic water-splitting technology.

Ab Initio Calculations for the Electronic, Interfacial and Optical Properties of Two-Dimensional AlN/Zr2CO2 Heterostructure

Recently, expanding the applications of two-dimensional (2D) materials by constructing van der Waals (vdW) heterostructures has become very popular. In this work, the structural, electronic and optical absorption performances of the heterostructure based on AlN and Zr2CO2 monolayers are studied by first-principles simulation. It is found that AlN/Zr2CO2 heterostructure is a semiconductor with a band gap of 1.790 eV. In the meanwhile, a type-I band structure is constructed in AlN/Zr2CO2 heterostructure, which can provide a potential application of light emitting devices. The electron transfer between AlN and Zr2CO2 monolayer is calculated as 0.1603 |e| in the heterostructure, and the potential of AlN/Zr2CO2 heterostructure decreased by 0.663 eV from AlN layer to Zr2CO2 layer. Beisdes, the AlN/Zr2CO2 vdW heterostructure possesses excellent light absorption ability of in visible light region. Our research provides a theoretical guidance for the designing of advanced functional heterostructures.

A first-principles study of two-dimensional NbSe2H/g-ZnO van der Waals heterostructures as a water splitting photocatalyst

Based on first-principles calculations, we propose a new two-dimensional (2D) van der Waals (vdW) heterostructure that can be used as a photocatalyst for water splitting. The heterostructure consists of vertically stacked 2D NbSe₂H and graphene-like ZnO (g-ZnO). Depending on the stacking orders, we identified two configurations that have high binding energies with an energy band gap of >2.6 eV. These 2D systems form a type-II heterostructure which enables the separation of photoexcited electrons and holes. The presence of a strong electrostatic potential difference across the 2D NbSe₂H and g-ZnO interface is expected to suppress the electron-hole recombination leading to an enhancement in the efficiency of the photocatalytic activity. Our study also shows that the 2D NbSe₂H/g-ZnO vdW heterostructure has good thermodynamic properties for water splitting. Furthermore, the optical absorption of the 2D NbSe₂H/g-ZnO vdW heterostructure extends into the visible light region. Our results suggest that the 2D NbSe₂H/g-ZnO vdW heterostructure is a promising photocatalytic material for water splitting.

Superior tunable photocatalytic properties for water splitting in two dimensional GeC/SiC van der Waals heterobilayers

The photocatalytic characteristics of two-dimensional (2D) GeC-based van der Waals heterobilayers (vdW-HBL) are systematically investigated to determine the amount of hydrogen (H2) fuel generated by water splitting. We propose several vdW-HBL structures consisting of 2D-GeC and 2D-SiC with exceptional and tunable optoelectronic properties. The structures exhibit a negative interlayer binding energy and non-negative phonon frequencies, showing that the structures are dynamically stable. The electronic properties of the HBLs depend on the stacking configuration, where the HBLs exhibit direct bandgap values of 1.978 eV, 2.278 eV, and 2.686 eV. The measured absorption coefficients for the HBLs are over ~ 105 cm-1, surpassing the prevalent conversion efficiency of optoelectronic materials. In the absence of external strain, the absorption coefficient for the HBLs reaches around 1 × 106 cm-1. With applied strain, absorption peaks are increased to ~ 3.5 times greater in value than the unstrained HBLs. Furthermore, the HBLs exhibit dynamically controllable bandgaps via the application of biaxial strain. A decrease in the bandgap occurs for both the HBLs when applied biaxial strain changes from the compressive to tensile strain. For + 4% tensile strain, the structure I become unsuitable for photocatalytic water splitting. However, in the biaxial strain range of - 6% to + 6%, both structure II and structure III have a sufficiently higher kinetic potential for demonstrating photocatalytic water-splitting activity in the region of UV to the visible in the light spectrum. These promising properties obtained for the GeC/SiC vdW heterobilayers suggest an application of the structures could boost H2 fuel production via water splitting.

Two-Dimensional As/BlueP van der Waals Hetero-Structure as a Promising Photocatalyst for Water Splitting: A DFT Study

Constructing van der Waals (vdW) hetero-structure by stacking different two-dimensional (2D) materials has become an effective method for designing new-type and high-quality electronic and optoelectronic nano-devices. In this work, we designed a 2D As/BlueP vdW hetero-structure by stacking monolayer arsenene (As) and monolayer blue phosphorous (BlueP) vertically, which were recently implemented in experiments, and investigated its structural, electronic, and photocatalytic water splitting properties by using the standard first principles calculation method with HSE06 hybrid exchange-correlation functional. Numerical results show that the As/BlueP vdW hetero-structure is structural robust, even at room temperature. It presents semi-conducting behavior, and the conduction band minimum (CBM) and the valence band maximum (VBM) are dominated by BlueP and As, respectively. The typical type-II band alignment predicts the potential application of the hetero-structure in highly efficient optoelectronics and solar energy conversion. Moreover, the CBM and the VBM straddle the redox potentials of water in acid environment, predicting the possibility of the As/BlueP hetero-structure as a 2D photocatalyst for water splitting. When an in-plane strain is applied, the band edges and, further, the optoelectronic properties of the hetero-structure can be effectively tuned. Especially, when tensile strain is equal to 4.5%, the optical absorption spectrum is effectively broadened in a visible light region, which will largely improve its photocatalytic efficiency, although the pH value of the solution range reduction. This work provides theoretical evidence that the As/BlueP hetero-structure has potential application as a 2D photocatalyst in water splitting.